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New Member

No problems selling power to the grid in Australia, it's already organised, i want to build my own & have it approved.
Can't justify buying one of the highly priced units on the market, i would rather save the money than buy them a Porsche, they are way over priced for what they are.

Well-Known Member

Eh Burnt,
What your proposing with your nic is frying a poor electrician working on a line and when the grid goes down he gets zapped by your homemade circuit. The rules are there for a reason. As you say

Will cost you more than a 25 kW solar/ grid tie array from a reputable supplier. If your that cheap and can't afford to go the right route for grid/tie just stay on the grid and pay ya carbon tax when its due.

" NEVER argue with stupid people, they will drag you down to their level and then beat you with experience"- Mark Twain

Super Moderator

I suspect you have no idea how much it might cost to get such a device approved, you would probably have to manufacture multiples of them, send them away to be tested (some probably to destruction), along with a LARGE cheque.

Well-Known Member

You'll also have to produce a fair surplus of electricity above and beyond your needs. Plus it's unlikely the power company will pay you the same amount as they charge you for your surplus.
Let's say you continously generate a surplus of 1kW per hour every hour of every day for a year (actually that's impressive) and the power company pays you $0.02 per hour for it. You'll make $175.35 per year. Call your power company and find out how much they'll pay you per kWh.

New Member

Gday bryan1
Not sure what your saying about frying a poor sparky, can't happen with the "correct" setup, i have no intention of building a cheap slapped up inverter.
It's not a matter of being cheap considering the R&D on the 10Kw Biomass generator i have built has cost plenty in money & time & hair loss but has been worth it & the unit has been "officially" approved, i am a Mechanical Engineer by trade..

Would you pay twenty times more for something than what it is worth in reality & more than likely it will be cheap Chinese rubbish that will not last passed on as quality with the likelyhood of frying everybody or catching fire & so on?

reputable supplier--ha!, show me one that doesn't put the dollar or themselves first,been there done that!

I see you live in the Adelaide hills so money is no issue.
For the rip off prices there charging for these units i would rather build my own & have it approved for the same money or less, at least that way i learn something useful along the way & put the money to good use.

Nigel Goodwin
I can get a single unit passed for $5000.00 that's half of what they want for a 10Kw unit. Yes they want to test it to destruction, i have all the necessary preliminaries out of they way.

blueroomelectronics
The contract i have with a particular electricity retailer is for $0.10 per Kwh X 10kWh X 24hrs x 365 days---seems worth it to me.
Over 10kWh the rate drops to $0.05 per kWh because you are not classifed as a small scale generation plant & are above there limit.

Well-Known Member

Considering how unlikely finding schematics for the inverter are and the cost of designing it yourself 10k for a 10kw unit doesn't seem unreasonable.
Assuming your 10kw generator is generating the full 10kw 100% of the time you'll earn $876.6 per year before taxes.
Try eBay maybe they sell used inverters.

New Member

Considering how unlikely finding schematics for the inverter are and the cost of designing it yourself 10k for a 10kw unit doesn't seem unreasonable.
Assuming your 10kw generator is generating the full 10kw 100% of the time you'll earn $876.6 per year before taxes.
Try eBay maybe they sell used inverters.

Banned

If so an easy way to do it, is to use an induction motor. First the motor is connected to the mains and run up to speed, then you connect the source of mechanical power to it using a clutch. As long as the shaft speed of the motor is faster than the synchronous speed, then the motor will act as an alternator pumping electricity into the grid rather than taking power from it. You need to monitor the shaft speed and shut down the system if it drops below the synchronous speed, otherwise you'll be taking more energy than you're producing.

If it's a DC source another cheap and inefficient way is to use a motor generator set. This is basically the same idea except a DC motor drives the induction motor/alternator. Again, you need to shut it down when the shaft speed drops below the synchronous speed.

A synchronous motor can be used for both of the above methods but the shaft speed will not change and I imagine this would make it harder to monitor. Rather than the speed changing the applied torque will change. I imagine that you could use some sort of strain gauge on the shaft or montor to find our whether it's acting as a motor or an alternator.

The third option is an electronic inverter. I've never seen a schematic for a grid tie inverter before. My only guess is that it uses a similar method as the motor but does it electronically. First the oscillator frequency is synchronised with the mains, then it's run as a constant current source with the output voltage fixed at the same as the mains and input power determines the output current and if there isn't enough input power it's shut down.

I imagine that a simple mechanical method is probably easier to gain approval for than a more complex electronic method.

I have no experiance in designing grid-tie inverters or any power generation system connected to the grid. The above post is all I know about the subject from reading forum.

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

Member

If you have a DC motor running a 3 phase generator, you can synchronize the phase voltages between the generator and the main grid by using 3 incandescent light bulbs and a suitable knife switch or breaker.... one light bulb being connected between the identical phase lines of each system. When the two systems are not in phase, the bulbs will glow. If the bulbs are glowing, you bump the rpm of the DC driver motor until there is no current flowing in the bulbs, and they are dark. This is more or less a trial and error technique. It doesn't take much effort though. When the bulbs are dark, you can throw the knife switch and the two systems will be synchronous, and lock in phase, within reasonable operating limits... If the generator provides power to the grid, everything is fine. However, if the generator absorbs power from the grid, it defeats the purpose, acting as a motor.

You will probably need a way to observe the power flow from the generator to the main system....This can be done with a special 3 phase power meter, or with two single phase watt meters.

You might want to check with your local power company to see what they say about this method ... Your efforts might affect the local power grid.

Member

I've tried modifying a 150w inverter (stand alone one). The main problem is to make it synchronise with the grid and disconnect when no grid is available. I used an inverter that contained two 494 PWM chips, basically one is used to control the conversion from 12v DC into 360v DC, then the second generates a modified sinewave (which isn't ideal) at 50Hz. I added a 50Hz ish sinewave into the CT pin and observed the output waveform. the output frequency changed with the over riding sinewave so ion theory it works. But when I connected a step-down transformer to the CT pin, it went up in smoke.

I've so far got through 3 different inverters and two 1400VA UPSs trying to learn what's going wrong.

The other problem with using the modified sine inverter is the harmonics that are introduced onto the grid, I don't think is such a big problem at such a small scale but could interfere with other equipment connected to the grid.

This project was thought up so i could plug anything renewable into it such as solar, wind, hydro, exercise bike etc. and feed onto the grid. I ended up just keeping the system isolated from the grid, charging batteries with what I've got and running appliances from the stand alone inverter. This was far from ideal as some applinces don't like modi-sine and the system had to be monitored the whole time.

Banned

I've tried modifying a 150w inverter (stand alone one). The main problem is to make it synchronise with the grid and disconnect when no grid is available. I used an inverter that contained two 494 PWM chips, basically one is used to control the conversion from 12v DC into 360v DC, then the second generates a modified sinewave (which isn't ideal) at 50Hz. I added a 50Hz ish sinewave into the CT pin and observed the output waveform. the output frequency changed with the over riding sinewave so ion theory it works. But when I connected a step-down transformer to the CT pin, it went up in smoke.

A standalone inverter is a constant voltage source. A grid-tie inverter is a constant current source. Just being in sync with the mains is not good enough, if the inverter is configured as a constant voltage source it will either overheat or shutdown.

I've so far got through 3 different inverters and two 1400VA UPSs trying to learn what's going wrong.

The other problem with using the modified sine inverter is the harmonics that are introduced onto the grid, I don't think is such a big problem at such a small scale but could interfere with other equipment connected to the grid.

By the way cutting off the inverter when the mains fails isn't too hard to do. If the inverter is configured as a constant current source the output voltage will track the mains. If there's a power cut the inverter will try to power the whole grid so its output voltage will drop to near zero. All you need to do is cut-off the inverter when the mains voltage drops below a certain threshold.

It's a shame because designing a grid tie inverter sounds would be a fun and challenging project. You could legally do it yourself if you powered your house from a diesel generator and a renewable power source and not be connected to the grid.

Last edited: Jan 23, 2009

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

Well-Known Member

Member

So is modifying a stand-alone inverter to connect to the grid the way to go or start completely from scratch?
I'm thinking that the SMPS and protection circuitry is already built in to a stand-alone unit, but is it too different or will need extensive modification to make it work?

I've seen a small grid-tie inverter (the Mastervolt Soladin 120) but they don't make it anymore and I can't get one anywhere. I'm thinking that if it can be made that small, a 150W inverter can do the same thing?